The prior art includes systems for detecting defects and/or irregularities on a web of material being conveyed along a measurement plane that are equipped with a device for emitting linear light. The term “linear light” is understood here to mean bundled light that produces an illuminated line on the web of material, with the line usually running transversely to the web. Such a system can be used for example to examine a web of paper that is being conveyed along the measurement plane at high speed, one or more line scan cameras preferably being used in the reflected light method or the transmitted light method to pinpoint the edges of the web or defects and/or irregularities (e.g. thin spots, the inclusion of flies or dirt particles, etc.) by detecting brightness differences on the web of material. In the reflected light method, both the emission apparatus and the camera are disposed on the same side of the web of material, and in the transmitted light method they are on different sides of the web.
Apparatuses for emitting linear light are known for example from WO 2004/006560 and from U.S. Pat. No. 6,880,952 B2.
An object of the present disclosure is to create an apparatus for emitting linear light and thus a measuring arrangement which are of compact and robust construction. A further aim is to be able to obtain as many illumination variants as possible with one apparatus.